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Progress in development of high capacity magnetic HTS bearings
Physica C 426–431 (2005) 739–745 www.elsevier.com/locate/physc
Progress in development of high capacity magnetic HTS bearings P. Kummeth *, W. Nick, H.-W. Neumueller Siemens AG, CT PS 3, Paul-Gossen-Str. 100, D-91052 Erlangen, Germany Received 23 November 2004; accepted 9 February 2005 Available online 5 July 2005
Abstract HTS magnetic bearings are inherently stable without an active feedback system. They provide low frictional losses, no wear and allow operation at high rotational speed without lubrication. So they are very promising for use in motors, generators and turbines. We designed and constructed an HTS radial bearing for use with a 400 kW HTS motor. It consists of alternating axially magnetized permanent magnet rings on the rotor and a segmented YBCO stator. Stator cooling is performed by liquid nitrogen, the temperature of the stator can be adjusted by varying the pressure in the cryogenic vessel. At 68 K maximum radial forces of more than 3.7 kN were found. These results range within the highest radial bearing capacities reported worldwide. The encouraging results lead us to develop a large heavy load HTS radial bearing. Currently a high magnetic gradient HTS bearing for a 4 MVA synchronous HTS generator is under construction. Ó 2005 Elsevier B.V. All rights reserved. PACS: 74.72.Bk; 85.25.Ly Keywords: Bearing; HTS; YBCO; NdFeB
1. Introduction Several advantages make magnetic HTS bearings promising candidates for application in rotat* Corresponding author. Tel.: +49 9131 7 34254; fax: +49 9131 7 33323. E-mail address: [email protected] (P. Kummeth).
ing machines with high operational speed. Their low friction, intrinsic safety, great reliability and lifetime offer technical and economic benefits. Simultaneously they provide radial and axial bearing capability. A number of small experimental bearings has been built and investigated in the recent years [1–4]. Our experimental results from static and dynamic investigations on journal type HTS test bearings (51 mm in diameter) [5],
0921-4534/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.02.074
740
P. Kummeth et al. / Physica C 426–431 (2005) 739–745
Fig. 1. Segments of polycrystalline melt textured YBCO manufactured by ATZ.
encouraged us to develop an HTS bearing for higher loads. In collaboration with Siemens A&D and external partners we have designed, manufactured and tested a superconducting synchronous 400 kW HTS motor [6]. Within the framework of this project the development of a magnetic high gradient HTS bearing for the 400 kW motor was carried out. We aimed for a radial bearing capacity of 2500 N. A modular assembly of the HTS bearing was chosen. The stator was made from polycrystalline melt textured YBCO as shown in Fig. 1, which was manufactured by Adelwitz Technologiezentrum GmbH (ATZ) [7]. It has a length of 250 mm, an inner diameter of 203 mm, is mounted in a simple low cost cryostat and cooled with liquid nitrogen. The rotor consists of 26 NdFeB permanent magnet rings with an outer diameter of 200 mm, which were mounted with alternating polarity, separated by iron disks for flux guidance on a shaft.
tometer. At 77 K and 0.5 T a jc of only 4.2 kA/cm2 was found. More experimental details are reported elsewhere [8]. The results gave evidence that there is potential to increase jc, because the critical current density was significantly lower than in smaller YBCO samples manufactured earlier. Further a cylinder-like a–b texture of the YBCO stator is necessary. Improvement of YBCO quality and grain alignment can be achieved by using a top-seeded-melt-growth process [9] for HTS preparation. NdFeB is the permanent magnet material offering the highest energy density currently available. The suitability of these rare earth magnets for use at low temperatures had already been demonstrated [5]. We used NdFeB magnet rings with an outer diameter of 200 mm, a photograph is shown in Fig. 2. Iron shims are located between the magnet rings to guide the magnetic flux. Rotational losses in HTS bearings can be caused by several effects. Inhomogeneities in rotating permanent magnet rings induce hysteretic losses in the superconductor and eddy current losses in metallic structures. On the other hand inhomogeneities in the superconducting stator will cause iron losses
2. Steps towards useful HTS bearings An HTS bearingÕs capacity is determined by HTS quality and performance of the permanent magnets. Because of unexpected low bearing capacities for the first two stators the critical current density jc of polycrystalline melt textured YBCO material of stator 2 was characterized by magnetization measurements in a SQUID magne-
Fig. 2. NdFeB magnet ring with an outer diameter of 200 mm, an inner diameter of 150 mm and a thickness of 8 mm, manufactured in one piece.
P. Kummeth et al. / Physica C 426–431 (2005) 739–745
Fig. 3. Characterization of circumferential radial magnetic flux homogeneity.
in the rotating permanent magnets and shims. Fig. 3 displays a test set-up to investigate the radial magnetic flux homogeneity of large permanent magnet rings. Two permanent magnet rings with alternating polarity, separated by non-magnetic spacers or magnetic flux guidance rings of various thicknesses were mounted on a rotating axle. The radial magnetic flux was measured by a hall-effect probe in about 1.2 mm distance. Magnet cooling during tests at 77 K was carried out by a bath filled with liquid nitrogen. Fig. 4 represents results from
Fig. 4. Radial magnetic flux profile of two magnet rings with alternating polarity separated by 2 mm G-FRP.
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tests with 2 mm G-FRP in-between the magnet rings at 293 K and 77 K. Operation at reduced temperature increased the magnetic induction of the magnets. A variation of the magnetÕs radial flux density in circumferential direction by about 10% was found at 77 K. Optimized magnetic shims between the magnets are able to smooth the flux variation. For a given quality of HTS and permanent magnets up-scaling of a bearingÕs load capacity means up-scaling in dimensions. Our HTS stator consists of YBCO segments which were glued into a copper hollow cylinder with epoxy resin. It is well known that granular HTS stators with a grain size greater than or equal the magnetic pole length (10 mm in our bearing) are suitable for HTS bearings. Detailed theoretical investigations revealed that smaller grains are acceptable, as long as jc is large enough. According to Ref. [10] a critical current density of 20 kA/cm2 at a magnetic induction of about 0.5 T seems to be sufficient for our magnetic HTS bearing. Processing of HTS stators with larger dimensions is regarded as practicable. The maximum feasible bearing diameter is determined by the available permanent magnet rings if use of segmented magnets is avoided. NdFeB is a very brittle material. To our knowledge NdFeB magnets with an outer diameter of 200 mm are the largest rare earth magnets manufactured in one piece up to now. Due to our design, a small bearing gap filled with gas, we cannot avoid that the magnets reach temperatures down to nearly 77 K. We proved experimentally, that the 200 mm magnet rings withstand rapid cooling in liquid nitrogen and rotation at 1800 rpm and 77 K. Investigation of the tensile strength of NdFeB revealed 100 MPa at 295 K and 112 MPa at 77 K, respectively. According to rough estimations a maximum circumferential velocity of 118 m/s is acceptable. This corresponds to more than 11000 rpm at a radius of 0.1 m. To achieve an optimized bearing capacity the magnetic pole width of the rotor has to be adapted to the bearing gap, i.e. the distance between YBCO stator and permanent magnet rings. An optimum bearing gap is about 0.1–0.2 times the pole width [11]. The gap of our HTS bearing was designed to 1.5 mm after cooling down, which is 0.15 times
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P. Kummeth et al. / Physica C 426–431 (2005) 739–745
Fig. 5. Permanent magnet rings are axially magnetized and stacked with alternating polarity on a former that is mounted on the shaft.
Fig. 6. HTS bearing with shaft mounted on a test device.
the pole width of 10 mm (thickness of magnets: 8 mm, thickness of iron disks: 2 mm). Fig. 5 presents the arrangement of the magnet rings and iron shims. This magnet module is mounted on the rotating shaft and is enclosed in the bearing housing in Fig. 6.
3. Experimental Bearing tests were performed in a test set-up to allow characterization of HTS motor and HTS bearing separately. Characterization of the bearing was performed with three YBCO stators of
Fig. 7. Cool down cycle of the test bearing.
different quality [12] and for several magnet configurations at stator temperatures between 66 K and 86 K. Due to improvements in the preparation process the quality increases from stator 1 to stator 3. Detailed analysis of the critical current density was carried out only for YBCO material of stator 2 [8]. A typical cool down cycle of the magnetic HTS bearing is plotted in Fig. 7. Up to nine Pt100 were used to monitor the statorÕs temperature at front and at center position and the magnetÕs temperature, too. After two hours the HTS stator has reached a homogeneous quasi-equilibrium temperature of about 80 K. The temperature of the permanent magnets was 86 K. Due to this small temperature difference heat transfer between rotor and stator via radiation is negligible. Characterization of the bearing properties was performed for several different magnet configurations to verify the design of our magnet module. We carried out these measurements at a stator temperature of approximately 80 K without rotation of the shaft using the YBCO stator no. 2. At first the bearing capacity of our primary bearing design, an alternating stack of NdFeB magnet rings with 8 mm thickness and iron shims with 2 mm thickness, was examined. In a next step, configurations with double magnets, i.e. an effective magnet thickness of 16 mm, and iron shims of 2 mm and 4 mm thickness were investigated. Finally we measured the bearing capacity for a stack of 8 mm magnets separated by spacers of 5.3 mm G-FRP. The experimental results are presented
P. Kummeth et al. / Physica C 426–431 (2005) 739–745
Fig. 8. Bearing capacity for various magnet configurations. Maximum capacity was found for (bearing gap/magnetic pole width) = 0.15, where magnets and iron disks have a thickness of 8 mm and 2 mm, respectively. These results are in good accordance to theoretical estimations that expect highest capacities for (bearing gap/magnetic pole width) 0.15–0.2.
in Fig. 8. As expected, our investigations revealed the highest capacity for the primary bearing design, where the ratio of bearing gap to magnetic pole width is 0.15. A lower ratio results in less load capacity. A significant reduction is found in the experiment with G-FRP spacers between the magnets, however. In this case the magnetic flux of the magnets is neither guided nor smoothed by the non-magnetic G-FRP. We found a significant temperature dependence of the bearing capacity. Operation at reduced temperature of the HTS stator results in enhanced bearing capacities. This behavior is caused by increased critical current densities at lower temperatures in YBCO and is a hint, that further improvement of the stator quality is still possible and necessary. Details about the temperature dependence of the critical current density of stator 2 are reported in Ref. [8]. Rotation of the shaft leads to reduced load capacity. Reason for this is a raise of the statorÕs temperature, which is caused by several effects. Hysteresis losses in the HTS stator and eddy current losses in metallic structures are caused by inhomogeneities in the permanent magnet rings. Inhomogeneities in the YBCO segments lead to losses in magnets and iron shims. Beside these effects our bearing gap is filled with helium gas to
743
Fig. 9. Temperature dependence of bearing capacity (measured for the shaft in center position) for various magnet configurations and different stators. The best results were obtained with stator no. 3.
avoid condensation of moisture and icing. Under rotation a turbulent gas circulation enhances the heat transfer from the shaft to the inner side of the YBCO stator. Especially the inner YBCO layer next to the rotor is warmed up and loses performance. More experimental details on HTS bearing losses under rotation are given in Ref. [8].
Fig. 10. Vibration of shaft under bearing operation at 1500 rpm (25 Hz) and 69 K.
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P. Kummeth et al. / Physica C 426–431 (2005) 739–745
At the motorÕs nominal speed of 1500 rpm (25 Hz) a radial bearing capacity of 2700 N was measured for a stator temperature of 72 K and the rotating shaft in centre position. Results of various experiments are plotted in Fig. 9. The highest bearing capacities were measured using stator no. 3. A rough estimation resulted in a resonance frequency of about 19 Hz for our HTS bearing in its test set-up. During operation of the bearing under rotation with 25 Hz (1500 rpm, corresponds to nominal speed of HTS motor) at a temperature of 69 K motion of the shaft was monitored contactless. We found an oscillation of the shaft around ±0.1 mm in horizontal (x) and vertical (y) direction as shown in Fig. 10.
4. Conclusions Our goal was to prove the feasibility of heavy load HTS radial bearings for large machines. Despite the low rotational speed (1500 rpm) the motor project was used to gather experience in development of large HTS bearings. Of course HTS bearings can demonstrate their benefits especially in rotating machines with high operational speed. For example slide bearings in turbines have bearing losses of several tens of kilowatts. Application of HTS bearings could reduce these losses significantly. The intrinsic safety of HTS bearings has two reasons. On the one hand HTS bearings need no control and therefore one does not have to care about electromagnetic compatibility or malfunction of a controller. Otherwise upon loss of cooling there is enough time to shut down the machine due to the large thermal inertia of the HTS stator. We have successfully designed, constructed and tested a magnetic high gradient HTS bearing for a 400 kW synchronous HTS motor. The nominally designed bearing capacity of 2500 N was exceeded: in operation with subcooled liquid nitrogen at T = 72 K and rotation of the shaft at 1500 rpm (corresponding to nominal speed of the motor) a radial bearing capacity of 2700 N was found for the shaft at centre position. Without rotation
of the shaft a capacity of 3700 N was determined. The load capacity of our bearing ranges within the highest radial bearing capacities reported worldwide. Further improvement of the bearing capacity can be achieved by enhanced critical current densities in the melt textured YBCO. A vacuum encapsulation of the YBCO stator is desired for future bearings to achieve homogeneity of temperature distribution throughout the entire HTS stator. Additionally bearing losses under rotation of the shaft caused by heat transfer via gas in the bearing gap would be reduced by a vacuum insulation. This is also very important to minimize the influence of flux creep. Experiments on small HTS bearings with 51 mm diameter revealed a strong dependence of flux creep on operation temperature as shown in Fig. 11. Cryofree cooling of HTS bearings is highly desirable and can be performed using the cold head of a cryocooler. This enables continuous operation below 77 K at a temperature of about 65 K. Our results indicate that a 5 kN bearing at 3600 rpm is achievable. An HTS bearing for a 4 MVA synchronous generator is currently under construction. Completion of the bearing and first tests are scheduled in 2005.
Fig. 11. Temperature dependence of flux creep rate investigated on a test bearing with 51 mm stator diameter. The stator had four Pt100 sensors. Mean temperature of each sensor is given by a symbol. The statorÕs mean temperatures are indicated by arrows.
P. Kummeth et al. / Physica C 426–431 (2005) 739–745
Acknowledgements The authors would like to thank O. Batz and P. Massek for assembly of the HTS bearing, W. Herkert for assistance during bearing characterization and V. Hussennether for characterization of YBCO samples in a SQUID magnetometer. Our work was supported by the German Bundesministerium fu¨r Bildung, Wissenschaft, Forschung und Technologie (BMBF) under grant number FKZ: 13N7621/8. References [1] W. Gawalek, T. Habisreuther, T. Strasser, M. Wu, D. Litzkendorf, K. Fischer, P. Go¨rnert, A. Gladun, P. Stoye, P. Verges, K.V. Ilushin, L.K. Kovalev, Appl. Supercond. 7/8 (1994) 465. [2] F. Werfel, R. Rothfeld, D. Wippich, U. Flo¨gel-Delor, Appl. Supercond. (1999); F. Werfel, R. Rothfeld, D. Wippich, U. Flo¨gel-Delor, Inst. Phys. Conf. Ser. 167 (2000) 1043. [3] J. Bock, M. Baecker, G. Brommer, L. Cowey, M. Kesten, H. Fieseler, W.R. Canders, H. May, H. Freyhardt, A.
[4] [5] [6]
[7]
[8] [9] [10] [11]
[12]
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Leenders, Appl. Supercond. (1999); J. Bock, M. Baecker, G. Brommer, L. Cowey, M. Kesten, H. Fieseler, W.R. Canders, H. May, H. Freyhardt, A. Leenders, Inst. Phys. Conf. Ser. 167 (1999) 1051. J.R. Hull, Supercond. Sci. Technol. 13 (2000) R1. P. Kummeth, W. Nick, G. Ries, H.-W. Neumu¨ller, Physica C 372–376 (2002) 1470. M. Frank, J. Frauenhofer, P. van Hasselt, W. Nick, H.-W. Neumueller, G. Nerowski, IEEE Trans. Appl. Supercond. 13 (2003) 2120. F.N. Werfel, U. Flo¨gel-Delor, D. Wippich, Appl. Supercond. (1997); F.N. Werfel, U. Flo¨gel-Delor, D. Wippich, Inst. Phys. Conf. Ser. 158 (1997) 821. P. Kummeth, G. Ries, W. Nick, H.-W. Neumueller, Supercond. Sci. Technol. 17 (2004) S259. S.O. Siems, W.-R. Canders, H. Walter, J. Bock, Supercond. Sci. Technol. 17 (2004) S229. C. Hofmann, G. Ries, Supercond. Sci. Technol. 14 (2001) 34. G. Ries, P. Kummeth, in: Proc. 6th Int. Symp. on Magnetic Suspension Technology, Turin, Italy, Politecnico di Torino, October 2001, p. 201. F.N. Werfel, U. Floegel-Delor, T. Riedel, R. Rothfeld, D. Wippich, B. Goebel, P. Kummeth, H.-W. Neumueller, W. Nick, IEEE Trans. Appl. Supercond. 13 (2003) 2173.
Progress in development of high capacity magnetic HTS bearings P. Kummeth *, W. Nick, H.-W. Neumueller Siemens AG, CT PS 3, Paul-Gossen-Str. 100, D-91052 Erlangen, Germany Received 23 November 2004; accepted 9 February 2005 Available online 5 July 2005
Abstract HTS magnetic bearings are inherently stable without an active feedback system. They provide low frictional losses, no wear and allow operation at high rotational speed without lubrication. So they are very promising for use in motors, generators and turbines. We designed and constructed an HTS radial bearing for use with a 400 kW HTS motor. It consists of alternating axially magnetized permanent magnet rings on the rotor and a segmented YBCO stator. Stator cooling is performed by liquid nitrogen, the temperature of the stator can be adjusted by varying the pressure in the cryogenic vessel. At 68 K maximum radial forces of more than 3.7 kN were found. These results range within the highest radial bearing capacities reported worldwide. The encouraging results lead us to develop a large heavy load HTS radial bearing. Currently a high magnetic gradient HTS bearing for a 4 MVA synchronous HTS generator is under construction. Ó 2005 Elsevier B.V. All rights reserved. PACS: 74.72.Bk; 85.25.Ly Keywords: Bearing; HTS; YBCO; NdFeB
1. Introduction Several advantages make magnetic HTS bearings promising candidates for application in rotat* Corresponding author. Tel.: +49 9131 7 34254; fax: +49 9131 7 33323. E-mail address: [email protected] (P. Kummeth).
ing machines with high operational speed. Their low friction, intrinsic safety, great reliability and lifetime offer technical and economic benefits. Simultaneously they provide radial and axial bearing capability. A number of small experimental bearings has been built and investigated in the recent years [1–4]. Our experimental results from static and dynamic investigations on journal type HTS test bearings (51 mm in diameter) [5],
0921-4534/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.02.074
740
P. Kummeth et al. / Physica C 426–431 (2005) 739–745
Fig. 1. Segments of polycrystalline melt textured YBCO manufactured by ATZ.
encouraged us to develop an HTS bearing for higher loads. In collaboration with Siemens A&D and external partners we have designed, manufactured and tested a superconducting synchronous 400 kW HTS motor [6]. Within the framework of this project the development of a magnetic high gradient HTS bearing for the 400 kW motor was carried out. We aimed for a radial bearing capacity of 2500 N. A modular assembly of the HTS bearing was chosen. The stator was made from polycrystalline melt textured YBCO as shown in Fig. 1, which was manufactured by Adelwitz Technologiezentrum GmbH (ATZ) [7]. It has a length of 250 mm, an inner diameter of 203 mm, is mounted in a simple low cost cryostat and cooled with liquid nitrogen. The rotor consists of 26 NdFeB permanent magnet rings with an outer diameter of 200 mm, which were mounted with alternating polarity, separated by iron disks for flux guidance on a shaft.
tometer. At 77 K and 0.5 T a jc of only 4.2 kA/cm2 was found. More experimental details are reported elsewhere [8]. The results gave evidence that there is potential to increase jc, because the critical current density was significantly lower than in smaller YBCO samples manufactured earlier. Further a cylinder-like a–b texture of the YBCO stator is necessary. Improvement of YBCO quality and grain alignment can be achieved by using a top-seeded-melt-growth process [9] for HTS preparation. NdFeB is the permanent magnet material offering the highest energy density currently available. The suitability of these rare earth magnets for use at low temperatures had already been demonstrated [5]. We used NdFeB magnet rings with an outer diameter of 200 mm, a photograph is shown in Fig. 2. Iron shims are located between the magnet rings to guide the magnetic flux. Rotational losses in HTS bearings can be caused by several effects. Inhomogeneities in rotating permanent magnet rings induce hysteretic losses in the superconductor and eddy current losses in metallic structures. On the other hand inhomogeneities in the superconducting stator will cause iron losses
2. Steps towards useful HTS bearings An HTS bearingÕs capacity is determined by HTS quality and performance of the permanent magnets. Because of unexpected low bearing capacities for the first two stators the critical current density jc of polycrystalline melt textured YBCO material of stator 2 was characterized by magnetization measurements in a SQUID magne-
Fig. 2. NdFeB magnet ring with an outer diameter of 200 mm, an inner diameter of 150 mm and a thickness of 8 mm, manufactured in one piece.
P. Kummeth et al. / Physica C 426–431 (2005) 739–745
Fig. 3. Characterization of circumferential radial magnetic flux homogeneity.
in the rotating permanent magnets and shims. Fig. 3 displays a test set-up to investigate the radial magnetic flux homogeneity of large permanent magnet rings. Two permanent magnet rings with alternating polarity, separated by non-magnetic spacers or magnetic flux guidance rings of various thicknesses were mounted on a rotating axle. The radial magnetic flux was measured by a hall-effect probe in about 1.2 mm distance. Magnet cooling during tests at 77 K was carried out by a bath filled with liquid nitrogen. Fig. 4 represents results from
Fig. 4. Radial magnetic flux profile of two magnet rings with alternating polarity separated by 2 mm G-FRP.
741
tests with 2 mm G-FRP in-between the magnet rings at 293 K and 77 K. Operation at reduced temperature increased the magnetic induction of the magnets. A variation of the magnetÕs radial flux density in circumferential direction by about 10% was found at 77 K. Optimized magnetic shims between the magnets are able to smooth the flux variation. For a given quality of HTS and permanent magnets up-scaling of a bearingÕs load capacity means up-scaling in dimensions. Our HTS stator consists of YBCO segments which were glued into a copper hollow cylinder with epoxy resin. It is well known that granular HTS stators with a grain size greater than or equal the magnetic pole length (10 mm in our bearing) are suitable for HTS bearings. Detailed theoretical investigations revealed that smaller grains are acceptable, as long as jc is large enough. According to Ref. [10] a critical current density of 20 kA/cm2 at a magnetic induction of about 0.5 T seems to be sufficient for our magnetic HTS bearing. Processing of HTS stators with larger dimensions is regarded as practicable. The maximum feasible bearing diameter is determined by the available permanent magnet rings if use of segmented magnets is avoided. NdFeB is a very brittle material. To our knowledge NdFeB magnets with an outer diameter of 200 mm are the largest rare earth magnets manufactured in one piece up to now. Due to our design, a small bearing gap filled with gas, we cannot avoid that the magnets reach temperatures down to nearly 77 K. We proved experimentally, that the 200 mm magnet rings withstand rapid cooling in liquid nitrogen and rotation at 1800 rpm and 77 K. Investigation of the tensile strength of NdFeB revealed 100 MPa at 295 K and 112 MPa at 77 K, respectively. According to rough estimations a maximum circumferential velocity of 118 m/s is acceptable. This corresponds to more than 11000 rpm at a radius of 0.1 m. To achieve an optimized bearing capacity the magnetic pole width of the rotor has to be adapted to the bearing gap, i.e. the distance between YBCO stator and permanent magnet rings. An optimum bearing gap is about 0.1–0.2 times the pole width [11]. The gap of our HTS bearing was designed to 1.5 mm after cooling down, which is 0.15 times
742
P. Kummeth et al. / Physica C 426–431 (2005) 739–745
Fig. 5. Permanent magnet rings are axially magnetized and stacked with alternating polarity on a former that is mounted on the shaft.
Fig. 6. HTS bearing with shaft mounted on a test device.
the pole width of 10 mm (thickness of magnets: 8 mm, thickness of iron disks: 2 mm). Fig. 5 presents the arrangement of the magnet rings and iron shims. This magnet module is mounted on the rotating shaft and is enclosed in the bearing housing in Fig. 6.
3. Experimental Bearing tests were performed in a test set-up to allow characterization of HTS motor and HTS bearing separately. Characterization of the bearing was performed with three YBCO stators of
Fig. 7. Cool down cycle of the test bearing.
different quality [12] and for several magnet configurations at stator temperatures between 66 K and 86 K. Due to improvements in the preparation process the quality increases from stator 1 to stator 3. Detailed analysis of the critical current density was carried out only for YBCO material of stator 2 [8]. A typical cool down cycle of the magnetic HTS bearing is plotted in Fig. 7. Up to nine Pt100 were used to monitor the statorÕs temperature at front and at center position and the magnetÕs temperature, too. After two hours the HTS stator has reached a homogeneous quasi-equilibrium temperature of about 80 K. The temperature of the permanent magnets was 86 K. Due to this small temperature difference heat transfer between rotor and stator via radiation is negligible. Characterization of the bearing properties was performed for several different magnet configurations to verify the design of our magnet module. We carried out these measurements at a stator temperature of approximately 80 K without rotation of the shaft using the YBCO stator no. 2. At first the bearing capacity of our primary bearing design, an alternating stack of NdFeB magnet rings with 8 mm thickness and iron shims with 2 mm thickness, was examined. In a next step, configurations with double magnets, i.e. an effective magnet thickness of 16 mm, and iron shims of 2 mm and 4 mm thickness were investigated. Finally we measured the bearing capacity for a stack of 8 mm magnets separated by spacers of 5.3 mm G-FRP. The experimental results are presented
P. Kummeth et al. / Physica C 426–431 (2005) 739–745
Fig. 8. Bearing capacity for various magnet configurations. Maximum capacity was found for (bearing gap/magnetic pole width) = 0.15, where magnets and iron disks have a thickness of 8 mm and 2 mm, respectively. These results are in good accordance to theoretical estimations that expect highest capacities for (bearing gap/magnetic pole width) 0.15–0.2.
in Fig. 8. As expected, our investigations revealed the highest capacity for the primary bearing design, where the ratio of bearing gap to magnetic pole width is 0.15. A lower ratio results in less load capacity. A significant reduction is found in the experiment with G-FRP spacers between the magnets, however. In this case the magnetic flux of the magnets is neither guided nor smoothed by the non-magnetic G-FRP. We found a significant temperature dependence of the bearing capacity. Operation at reduced temperature of the HTS stator results in enhanced bearing capacities. This behavior is caused by increased critical current densities at lower temperatures in YBCO and is a hint, that further improvement of the stator quality is still possible and necessary. Details about the temperature dependence of the critical current density of stator 2 are reported in Ref. [8]. Rotation of the shaft leads to reduced load capacity. Reason for this is a raise of the statorÕs temperature, which is caused by several effects. Hysteresis losses in the HTS stator and eddy current losses in metallic structures are caused by inhomogeneities in the permanent magnet rings. Inhomogeneities in the YBCO segments lead to losses in magnets and iron shims. Beside these effects our bearing gap is filled with helium gas to
743
Fig. 9. Temperature dependence of bearing capacity (measured for the shaft in center position) for various magnet configurations and different stators. The best results were obtained with stator no. 3.
avoid condensation of moisture and icing. Under rotation a turbulent gas circulation enhances the heat transfer from the shaft to the inner side of the YBCO stator. Especially the inner YBCO layer next to the rotor is warmed up and loses performance. More experimental details on HTS bearing losses under rotation are given in Ref. [8].
Fig. 10. Vibration of shaft under bearing operation at 1500 rpm (25 Hz) and 69 K.
744
P. Kummeth et al. / Physica C 426–431 (2005) 739–745
At the motorÕs nominal speed of 1500 rpm (25 Hz) a radial bearing capacity of 2700 N was measured for a stator temperature of 72 K and the rotating shaft in centre position. Results of various experiments are plotted in Fig. 9. The highest bearing capacities were measured using stator no. 3. A rough estimation resulted in a resonance frequency of about 19 Hz for our HTS bearing in its test set-up. During operation of the bearing under rotation with 25 Hz (1500 rpm, corresponds to nominal speed of HTS motor) at a temperature of 69 K motion of the shaft was monitored contactless. We found an oscillation of the shaft around ±0.1 mm in horizontal (x) and vertical (y) direction as shown in Fig. 10.
4. Conclusions Our goal was to prove the feasibility of heavy load HTS radial bearings for large machines. Despite the low rotational speed (1500 rpm) the motor project was used to gather experience in development of large HTS bearings. Of course HTS bearings can demonstrate their benefits especially in rotating machines with high operational speed. For example slide bearings in turbines have bearing losses of several tens of kilowatts. Application of HTS bearings could reduce these losses significantly. The intrinsic safety of HTS bearings has two reasons. On the one hand HTS bearings need no control and therefore one does not have to care about electromagnetic compatibility or malfunction of a controller. Otherwise upon loss of cooling there is enough time to shut down the machine due to the large thermal inertia of the HTS stator. We have successfully designed, constructed and tested a magnetic high gradient HTS bearing for a 400 kW synchronous HTS motor. The nominally designed bearing capacity of 2500 N was exceeded: in operation with subcooled liquid nitrogen at T = 72 K and rotation of the shaft at 1500 rpm (corresponding to nominal speed of the motor) a radial bearing capacity of 2700 N was found for the shaft at centre position. Without rotation
of the shaft a capacity of 3700 N was determined. The load capacity of our bearing ranges within the highest radial bearing capacities reported worldwide. Further improvement of the bearing capacity can be achieved by enhanced critical current densities in the melt textured YBCO. A vacuum encapsulation of the YBCO stator is desired for future bearings to achieve homogeneity of temperature distribution throughout the entire HTS stator. Additionally bearing losses under rotation of the shaft caused by heat transfer via gas in the bearing gap would be reduced by a vacuum insulation. This is also very important to minimize the influence of flux creep. Experiments on small HTS bearings with 51 mm diameter revealed a strong dependence of flux creep on operation temperature as shown in Fig. 11. Cryofree cooling of HTS bearings is highly desirable and can be performed using the cold head of a cryocooler. This enables continuous operation below 77 K at a temperature of about 65 K. Our results indicate that a 5 kN bearing at 3600 rpm is achievable. An HTS bearing for a 4 MVA synchronous generator is currently under construction. Completion of the bearing and first tests are scheduled in 2005.
Fig. 11. Temperature dependence of flux creep rate investigated on a test bearing with 51 mm stator diameter. The stator had four Pt100 sensors. Mean temperature of each sensor is given by a symbol. The statorÕs mean temperatures are indicated by arrows.
P. Kummeth et al. / Physica C 426–431 (2005) 739–745
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